The composite color video signals that are conventionally broadcast, for example in the NTSC (National Television System Committee) format, contain not only picture information (luminance and chrominance components) but also timing information (vertical sync pulses and horizontal sync pulses) and other reference information (e.g. equalizing pulses and color burst). Referring to FIG. 1 of the accompanying drawings, the horizontal sync pulse 2 and burst 4 both occur in the horizontal blanking interval, i.e., the interval between the active line times of consecutive horizontal scan lines. The horizontal sync pulse is a negative-going pulse having an amplitude of 40 IRE units, the 50 percent point 6 of the leading edge of the sync pulse being regarded as the horizontal sync point. Burst follows the horizontal sync pulse in the horizontal blanking interval and comprises a sinusoidal wave. The peak-to-peak amplitude of the burst is 40 IRE units, and immediately before and after the burst the signal is at blanking level (zero IRE). The burst ideally has a sine-squared envelope, and builds up from, and decays to, blanking level within one or two cycles of the burst wave. In accordance with EIA (Electronics Industries Association) standard RS 170 A, the start of burst is defined by the zero-crossing (positive or negative slope) that precedes the first half-cycle of subcarrier that is 50 percent or greater of the burst amplitude, i.e., 40 IRE. The color burst is used in the television receiver to control a phase-locked oscillator which generates a continuous wave at subcarrier frequency and is used to extract the chrominance information from the composite video signal.
Although the NTSC frame is made up of 525 lines which are scanned in two interlaced fields of 262.5 lines each, the NTSC color signal requires a four field sequence, and in order to facilitate matching between video signals from different sources, e.g. at the input to a production switcher, it is necessary to distinguish between the different fields of the four field sequence. Fields 1 and 2 can be distinguished on the basis of vertical sync information, but in order to distinguish field 1 from field 3 (or field 2 from field 4) it is necessary to consider SC/H (subcarrier to horizontal sync) phase. In accordance with standard RS 170 A, field 1 is characterized by the fact that a positive-going zero crossing of the extrapolated color burst (the continuous wave at subcarrier frequency and in phase with burst) on line 10 coincide with the sync point of that line. The pattern of sync and burst information for fields 1 and 3 is identical except for the phase of burst. Thus, in field 3, the negative-going zero crossing of the extrapolated color burst coincides with the sync point on line 10. Accordingly, in order to identify the different fields of the four field color sequence, and to adjust the SC/H phase so as to achieve the desired coincidence between the zero crossing point of the extrapolated color burst and the sync point, it is necessary to be able to observe the phase of the color burst relative to the sync point.
Several attempts have previously been made to measure SC/H phase. For example, using the Tektronix 1410 signal generator, it is possible to generate, in the middle of an unused line containing equalizing pulses, a wave at subcarrier frequency and in phase with burst. Since the leading edges of the equalizing pulses are midway between sync pulses, a measurement of subcarrier to horizontal sync phase can be implied by comparing the wave with the equalizing pulse timing. Alternatively the 1410 signal generator can generate a burst phased subcarrier during horizontal blanking which replaces a sync pulse and which can be compared with the remaining sync pulses. However, this equipment is not always available to technicians who need to make SC/H phase measurements. The Grass Valley Group 3258 SC/H phase meter provides a digital output of the phase difference between subcarrier and horizontal sync, but this again requires availability of dedicated equipment.
It is also known to measure SC/H phase using a dual trace oscilloscope having delayed sweep and the capability of inverting the input of one channel. The video signal and a CW signal at subcarrier frequency are applied to the oscilloscope in A plus B mode with the video signal inverted, and the phase of the CW signal is adjusted to achieve a null during burst of the video signal so that the CW signal is then in phase with burst. The oscilloscope is then adjusted to the chop mode and non-inverted video, and in this state the oscilloscope displays three traces, namely the waveform of the video signal, and two waveforms of the CW signal, triggered 180.degree. out of phase. The two waveforms of the CW signal cross at 0.degree. and 180.degree. and therefore the horizontal distance between the sync point and the nearest crossing point of the two waveforms of the CW signal is a measure of SC/H phase. This method of measuring SC/H phase requires use of equipment that might not be readily available to technicians, and suffers from the disadvantage that the oscilloscope display of the video waveform is contaminated with the two waveforms of the CW signal.
The waveform monitor, which provides an X-Y display of the amplitude of a video signal in the time domain, is commonly used by video engineers and technicians, but the conventional waveform monitor cannot be used to provide a reliable measurement of SC/H phase.